In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date part of common general knowledge, or known to be relevant to an attempt to solve any problem with which this specification is concerned.
Powered closures, such as sliding, roller or sectional doors, shutters, gates and the like are in common use for access control to areas, such as garages, car parks, private property, warehouses, factories, etc. Such closure arrangements are typically driven by operator units incorporating electric motors coupled to drive the closure between open and closed positions in an appropriate manner.
In the event of a power failure, fire or other unforeseen eventualities, the motor of such an operator unit may not be operable, and it is therefore desirable that manual operation of the closure be readily achieved independently of the motor. Indeed, for safety reasons it is often a requirement that selective manual operation be provided for powered closures.
For roller doors or shutters in industrial applications, in particular where the operator unit may be mounted at a significant height above the ground, manual operation of such closures may be achieved by way of an endless chain extending about an overhead chain pulley wheel. The pulley wheel engages with a shaft which in turn provides the drive to the door or shutter. The chain extends down to an area adjacent ground level where it may be pulled by an operator in order to effect movement of the door.
As the chain permanently resides on the chain pulley wheel, the chain pulley wheel cannot rotate during ordinary powered operation of the door or shutter, as a continuously rotating chain would provide a safety danger and risk derailment. It is therefore necessary to provide a clutch in the drive mechanism so that the chain pulley wheel only engages with the shaft when the door is to be manually operated.
Various clutch mechanisms have been proposed in the past. Most are highly complex, inconvenient to operate, or costly to manufacture and maintain.